This invention relates to an improved amplifier system with on-demand power supply boost and to a tracking charge pump useful in such a system.
Amplifier efficiency is ratio of power supplied by the amplifier to a load to the power supplied to the amplifier. The crest factor of an applied input signal to an amplifier is the ratio of the peak to RMS levels of that signal. It the crest factor is large enough, the efficiency of the system can be degraded. Further stated: input signals with a generally low RMS value and occasional high peak level cause an amplifier to operate at reduced efficiency. This is so because the power supply is supplying power at a voltage that is at all times sufficient to handle the infrequently occurring peaks and that power is being wasted when the peaks are absent and only the normal RMS levels are present at the input. There are several methods to improve this efficiency. In one approach there are provided a number of power supplies with a steering circuit that adds the current from the second higher potential power supply when the signal exceeds the capacity of the first. This can be extended to third and fourth supplies and beyond. One problem with this approach is that a number of power supplies must be provided. Further, since all of the power supplies have at least a quiescent current operating all the time, the practical efficiency is not greatly improved. Another approach employs the use of a synthesized output impedance in the form of positive feedback. While the efficiency of such a system is improved, the fact that the amplifier is still biased between power supplies substantially greater than the RMS level of the input signal results in less than optimal efficiency. A second approach involves sensing the amplifier""s output signal and boosting internal circuitry in a feedback approach. While feasible, this approach can be difficult to implement due to the delay in reacting to the output.
It is therefore an object of this invention to provide an improved tracking charge pump.
It is a further object of this invention to provide such a tracking charge pump which tracks the input to boost the power supply voltage only when needed.
It is a further object of this invention to provide an improved amplifier system with on-demand power supply boost.
It is a further object of this invention to provide such an improved amplifier system with on-demand power supply boost which uses less power and is more efficient.
It is a further object of this invention to provide such an improved amplifier system which requires only one set of power supply voltages.
It is a further object of this invention to provide such an improved amplifier system which avoids unsatisfactory delays.
The invention results from the realization that an improved charge pump for providing a voltage boost to a power supply can be achieved by tracking the input signal and applying a voltage boost from the charge pump only when needed and the further realization that an amplifier system with on-demand power supply boost can be achieved by tracking the input signal and applying a boost voltage from a charge pump to an amplifier only when the input signal level indicates that the normal power supply level will be insufficient.
This invention features a charge pump including a charge storage device and an isolation device interconnecting the charge storage device and one pole of a power supply. A driver circuit is responsive to an input signal and is interconnected between both poles of the power supply. The power supply provides predetermined power supply voltages. A bias circuit biases the driver circuit to charge the charge storage device in the charge mode and in the boost mode increases beyond the predetermined power supply voltages the supply voltage supplied by the storage device to an external load in response to an increase in the input signal. There may be an absolute value circuit responsive to the input signal for delivering the absolute value of the input signal to the charge pump.
In a preferred embodiment, the charge storage device may include a capacitor; the isolation device may include a switch or diode. The driver circuit may include a buffer circuit or transistor. The bias circuit may include a current source or resistor. The external load may include an amplifier and there may be a gain circuit for increasing the voltage applied by the storage device to the amplifier in response to an increase in the input signal as a function of the gain of the amplifier.
This invention also features an amplifier system with an on-demand power supply boost including an amplifier circuit, for receiving an input signal, and a charge pump connected to positive and negative power supplies and having predetermined supply voltages and is responsive to the input signal, for increasing beyond the predetermined supply voltages, the supply voltage applied to the amplifier in response to an increase in the input signal.
In a preferred embodiment, the charge pump may include a tracking charge pump. The tracking charge pump may include a charge storage device and a unidirectional isolation device interconnecting the charge storage device and one pole of the power supply. There may be a driver circuit responsive to an input signal and connected between both poles of the power supply. The power supply may have pre-determined power supply voltages. There may be a bias circuit for biasing the driver circuit to charge the charge storage device in the charge mode, and in the boost mode, for increasing beyond the predetermined power supply voltages, the supply voltage applied by the storage device to an external load in response to an increase in the input signal. There may be an absolute value circuit responsive to input signal for delivering the absolute value of the input signal to the charge pump. There may be a gain circuit for controlling the increase in voltage produced by the charge pump as a function of the gain of the amplifier. There may be a bias circuit for delivering a bias current to the charge pump. There may be a threshold circuit responsive to the input signal exceeding a predetermined threshold for actuating the bias circuit to provide a bias to the charge pump. The bias circuit may provide a bias boost current to the amplifier. There may be delay means for aligning the pump voltage and output signal.